The present invention is directed to a method for creating throughplating holes in a multilayer structure, or example, a multi-ply printed circuit board which has a layer sequence of metal-synthetic-metal layers.
In multi-ply printed circuit boards, the individual wiring levels are directly conductively connected to one another by means of so-called throughplatings. Miniaturization of the printed circuit boards is limited by, among other things, a space requirement for these throughplatings. The space requirement is determined by the area that must be reserved in the wiring level or layer for the throughplating as well as by the length of the throughplating. In length, the throughplating extends either through the entire thickness of the printed circuit board or through the entire thickness of a prefabricated single core within the printed circuit board.
It is known to mechanically drill the holes for the throughplatings. In this method, the space requirement of the throughplating is essentially co-defined by the drill diameter. It is not possible in mechanical boring to reliably produce blind holes which terminate on a metal layer lying in the printed circuit board. The minimum drilling diameter is limited by the drilling tool. Other methods of producing throughplating openings are therefore gaining in significance with increased miniaturization of the circuit boards.
It has also been known to utilize a focused CO.sub.2 laser for producing blind holes in a single-shot method, which blind holes are subsequently plated or filled with metal to form the connection between two metal layers. Such a method is briefly discussed in U.S. Pat. No. 4,030,190. In this method for producing a multilayered printed circuit, the bottom metal layer will serve for terminating or concluding the process of removing the layer of synthetic or plastic insulating layer.